Abstract

The tectonometamorphic histories of basement and cover units in the eastern Trans-Hudson Orogen attest to the importance of structural, geohydrological, and magmatic controls on the attainment of metamorphic conditions in a convergent-plate-margin setting. In northern Quebec, two metamorphic suites are recognized in the parautochthonous (lower-plate) Superior Province basement. An older metamorphic suite comprises arc-related, granulite-facies assemblages (less than 5 kbar and 860 to 920°C) dated at ca. 2.70 Ga. A younger, overprinting, collision-related metamorphic suite comprises amphibolite-facies assemblages (7.7 kbar at 640°C to 9.8 kbar at 715°C) dated at ca. 1.80 Ga. Within the overlying Paleoproterozoic Cape Smith Belt (lower-plate, south-verging thrust belt), thermal-peak mineral growth is syn- to post-thrusting; a relatively high-pressure, greenschist- to amphibolite-facies metamorphism (6.3 kbar at 400°C to 9.1 kbar at 575°C) is interpreted to be a result of ca. 1.80 Ga collision-related thickening. At higher structural levels, upper-plate mid-crust plutonic and metasedimentary units of the 1.86–1.82 Ga Narsajuaq arc contain granulite-facies assemblages (7–10 kbar and 800–900°C) retrograded to amphibolite-facies conditions (7–9 kbar and 700–775°C) during collision-related thrusting. At the structurally highest level, granulite-facies metamorphism of the (upper-plate) Lake Harbour Group and Blandford Bay assemblage is related to emplacement of the 1.86–1.85 Ga Cumberland batholith. Between ca. 1820 and 1795 Ma, accretion and collision-related thickening resulted in retrogression of the granulite-facies assemblages in the Narsajuaq arc, Lake Harbour Group and Blandford Bay assemblage, thermal peak metamorphism of lower-plate cover rocks, and re-equilibration of the parautochthonous Superior Province basement, all at greenschist- to amphibolite-facies conditions. The integration of field, thermobarometric and U–Pb data establishes a causal relationship between arc plutonism and development of granulite-facies assemblages in a convergent-plate-margin setting. In contrast, the attainment of greenschist- to amphibolite-facies conditions at mid-crust levels during plate collision is primarily a function of structural and geohydrological controls.